1. Field of the Invention
This invention pertains generally to video encoding methods, and more particularly to MPEG-2 encoder inter-quantization methods.
2. Description of Related Art
MPEG-2 is a multi-part video compression standard which has been adopted world-wide as a core technology for use in digital media distribution and playback in a variety of applications, which include digital television, computer, communications and consumer industries. MPEG-2 is an extension of the MPEG-1 international standard for digital compression of audio and video signals. MPEG-2 is directed at broadcast formats at higher data rates and provides extra algorithmic ‘tools’ for efficiently coding interlaced video, while supporting a wide range of bit rates and providing multichannel surround sound coding.
Recent progress in digital technology has made the widespread use of compressed digital video signals practical. The use of standardized compression allows new services to interoperate with each other and encourages necessary investments toward producing inexpensive integrated circuits and equipment.
MPEG-2 is a video coding standard supporting a diverse range of applications, and utilizing different algorithmic ‘tools’, by way of profiles and levels, which are implemented for each of a number of applications as defined within the full standard.
The color signals utilized by MPEG-2 compression are expressed as luminance (Y) and chrominance (U and V) components. The chrominance bandwidth may be reduced relative to the luminance without significantly affecting the picture quality. The terms 4:2:2 and 4:2:0 are often used to describe the sampling structure of the digital picture, with 4:2:2 indicating the chrominance is horizontally subsampled by a factor of two relative to the luminance, while 4:2:0 means the chrominance is horizontally and vertically subsampled by a factor of two relative to the luminance. Although other arrangements may be alternatively utilized.
MPEG-2 is capable of significantly compressing the bit rate of standard-definition 4:2:0 video, such as down to about 3-15 Mbit/s, although at lower bit rates quality suffers and artifacts are introduced. Compression is performed in response to removing redundant information from the signal at the coder prior to transmission and then re-inserting this information at the decoder. A coder and decoder pair are referred to as a ‘codec’.
Two types of redundancy are addressed by compression in modern video codecs, they are (1) spatial and (2) temporal. In each form of encoding correlations within the video are made use of to reduce bit rates of the resultant video. In reducing spatial redundancy, the relationships between neighboring pixels in the same vicinity of a frame are utilized to minimize the encoding rate and thus provide spatial compression. In reducing temporal redundancy, the temporal (time-related) relationships between pixels in nearby frames (temporally displaced images) are utilized to minimize encoding rate and thus provide temporal compression.
Two key techniques employed in an MPEG codec are intra-frame (within the frame) Discrete Cosine Transform (DCT) coding and motion-compensated inter-frame (between frames) prediction. It will be noted that “intra” is the prefix from Latin meaning “within”, while “inter” is a prefix from Latin which means “between”, or “amidst”.
During DCT coding a two-dimensional DCT is performed on small blocks (e.g., 8×8 pixels) of each component of the picture to produce blocks of DCT coefficients. The magnitude of each DCT coefficient indicates the contribution of a particular combination of horizontal and vertical spatial frequencies to the original picture block. The coefficient corresponding to zero horizontal and vertical frequency is called the DC coefficient.
Quantization is adjusted to reduce numerical precision while maintaining a desired level of perceived quality in the DCT blocks transmitted to the decoder, wherein the inverse transform is performed to reconstruct each image in the sequence of images. The degree of quantization applied to each coefficient is weighted according to the visibility of the resulting quantization noise to a human observer. It should be noted that in practice, this results in high-frequency coefficients being more coarsely quantized than low-frequency coefficients.
Coding is the process of serialization and encoding of the quantized DCT coefficients to exploit the likely clustering of energy into the low-frequency coefficients and the frequent occurrence of zero-value coefficients. For example, the block may be scanned in a diagonal zig-zag pattern starting at the DC coefficient to produce a list of quantized coefficient values, ordered according to the scan pattern.
One of the continuing drawbacks of MPEG-2 coding and decoding is that it remains computationally intensive, with quantization regarded as the most computationally expensive function within an encoder. High complexity computations require high-bandwidth processors to perform the coding and encoding in real-time, or extended processing times for off-line (non-real-time processing) operations.
Accordingly, a need exists for methods of reducing computational complexity when encoding video, such as according to the MPEG-2 standard. The present invention fulfills that need, and others, without diminishing the quality of encoder output.